The thermal, solvent‐free addition of maleic anhydride to methyl calendulate (methyl 8,10‐ trans ,12‐ cis ‐octadecatrienoate) ( 1a ) occurs with very high regio‐ and stereoselectivity at C‐8 and C‐11 of the fatty compound giving the endo ‐Diels–Alder adduct 4 with retention of the cis ‐configured double bond. Analogously, the Diels–Alder addition of maleic anhydride to methyl α‐eleostearate (methyl 9‐ cis ,11,13‐ trans ‐octadecatrienoate) ( 2a ) takes place regio‐ and stereoselectively at C‐11 and C‐14 yielding the endo ‐product 5 which was hydrolyzed to give the respective tricarboxylic acid 6 . X‐ray diffraction analyses of compounds 4 and 6 clearly show the regio‐ and stereoselectivity of the Diels–Alder addition reactions.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)
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Biermann et al. (2007) studied this question.
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